This invention relates generally to a method and apparatus for encoding and decoding an information signal and, in particular, is directed to a method and apparatus for encoding and decoding binary data words into a binary digital code of the same length.
Conventionally, apparatus for recording and reproducing a color video signal have been of the analog, rather than digital, type. However, due to progress in the field of high density digital recording, it has recently proved feasible to use digital video tape recorders (VTR) for recording and reproducing a digitized color video signal. With digital VTRs, an analog video signal is sampled every n bits for each digital word, for example, where n=8, and is thereby converted into a digital color video signal comprised of 8-bit words.
However, such 8-bit digital information signal generally includes a DC component, that is, the number of "1" or "0" bits is greater than the number of bits of the other type. Because conventional magnetic recording/reproducing apparatus is not responsive to direct current, any DC component of a transmitted or reproduced signal is likely to be lost.
A general indication of the DC content of a digital information signal is the digital sum variation (DSV) which is the instantaneous value of the integration of the digital information signal. If the DSV increases or decreases indefinitely, the digital information signal has a DC component, and if the DSV is limited or bounded, the digital information signal is DC free. In particular, the closer that the DSV is to zero, the fewer DC components exist in the signal.
In order to reduce the DSV, and thereby the DC content of an 8-bit digital information signal, various block encoding systems such as an 8-9 code conversion system, an 8-10 code conversion system and various other encoding systems, for example, M.sup.2 (modified Miller) encoding, and the like have been proposed. However, with the aforementioned encoding systems, the recording bit rate becomes greater than the source bit rate, that is, by expanding the length of each 8-bit data word to 9 bits, 10 bits, 16 bits or the like, the DSV of the digital information signal to be recorded is minimized at the expense of an increased recording bit rate. It is to be appreciated that this latter aspect is contrary to the desire that the recording bit rate be as low as possible to increase the amount of information that can be recorded on the record medium. In this regard, because of the desire to increase the information in a digital information signal, the source bit rate may increase, and accordingly, it is not practically possible to increase the recording bit rate. It is therefore desirable to reduce the DSV of the digital information signal to be recorded while maintaining the source bit rate for the encoded digital information signal to be recorded.